Flagella are composed of three parts: the basal body, hook and filament. The basal body is made up of 15 proteins with 4 rings (G-ves) or 2 (G+ves) which embed in the membrane. The hook acts a join and is made of a single protein. The protein is the hollow ‘tail’ of the flagella which is made of flagellin. This is very rigid and is able to spontaneously aggregate in the right ionic environment.

Structure of pili

In contrast, the structure of pili is less complex. Pili are straight rods made of pilin protein that also extend from the cell.

Mode of assembly of flagella

The first step in the assembly of flagella is that the basal body is assembled and embedded into the cell envelope. The hook is subsequently attached. Flagellin molecules are subsequently passed through the structure and are added at the tip to create the filament. Interestingly, there is a limit to how long the filament can be and some bacteria (such as E.coli) are able to limit its length.

Mode of assembly of pili

Pili are assembled by the addition of pillin proteins at the membrane edge (c.f. flagella where subunits are added at the tip)

Function of flagella

Flagella contribute to the motility of bacterial cells by rotating and acting as a bacterial motor. This process is driver by the re-entry of protons into the cell (which is like proton motive force in reverse).

Function of Pili

Pili have numerous functions including attachment, transfer of proteins and nucleic acids to other cells and motility. Pili motility is not provided by a rotating mechanism (like flagella) but instead by extension and subsequent depolymerisation and retraction. Pili are also an important antigen.

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Interested in correcting misuse of biology, helping people benefit from innovation.I teach courses in food science, food safety, biotechnology and microbiology at the University of Melbourne. My Bachelor Degree is in biochemistry and my Ph.D. applied molecular genetics. Thesis topic -- production of an animal feed supplement using genetically manipulated bacteria.---My recent publications have been on epidemics caused by bacterial pathogens, published in high-ranking journals. My current area of research is food risk analysis and management. The most important of these risks are microbes in food and microbial products that are toxic.